Several commercial product offerings known as video display processors (VDPs) or graphics display controllers are now available for either generating a graphics display or for processing a video signal so as to generate a graphics overlay which is superimposed on the picture represented by the video signal. When used in the overlay mode, the VDP is connected between a source of video signals and a display or broadcast device. The processor is arranged to receive a composite video signal as well as local or remotely generated data which defines the nature of the material to be overlaid, and to provide a video output in a standard NTSC format.
When a video disc player is used as a video source and connected directly to a video display processor, the resulting overlaid graphics have been found to be unsatisfactory, due primarily to jitter and noise in the sync pulses applied to the VDP. This jitter results from time base errors in the disc player output and noise from imperfections in the disc, and is noticed in several ways. First, horizontal jitter results in a varying horizontal scan rate (time base error) due to the player servo and recorded error, and causes all lines of the overlay to move left and right individually, making the characters appear wavy since the VDP does not track the jitter. The video picture from the disc appears to be stable on a video monitor, because the monitor itself corrects the disc video error by processing the disc's sync signals, while the overlay information is uncorrected and thus appears to move. Second, vertical jitter due to noise from the disc player being erroneously interpreted as horizontal sync pulses causes entire groups or all lines of the overlay to move up and down, making the characters appear to bounce. This problem, which becomes particularly noticeable when the disc player is in a still frame mode, results from the fact that large positive going noise spikes typically exist on only one field (e.g., odd) of the picture. Thus, when the graphics are overlaid, the relative position between the fields tends to change by one line, since the VDP's horizontal counters have an extra count for one field. The resulting upward movement of the overlay by one horizontal line space after a noise spike appears as up/down bounce, because the fields are interlaced.
A third problem occurs when excessive noise in the disc output is erroneously interpreted as a vertical sync pulse, causing the entire overlay to repeat at some point after the top of the picture. This phenomenon usually occurs when a large noise spike is adjacent to a horizontal sync pulse.
In view of the foregoing, it is the broad object of the present invention to provide an interface between a video signal source, such as a video disc player, and a video display processor which permits a picture to be overlaid with graphics without annoying effects caused by jitter and noise. Specific objects are the provision of a hardware interface circuit that corrects problems in sync pulses applied to the VDP from a video disc player and provides a clock input for the VDP which tracks the jitter.